The present invention relates to a circuit for adapting a headset comprising an ear piece and a microphone to a telephone handset. In particular, the invention relates to an electrical interface circuit to efficiently couple a headset microphone signal to a telephone signal transmitting circuit.
A considerable market has developed for adaptations and attachments to the commercially-available telephone handset. Among theses adaptations are telephone headsets which may be clamped to a user""s head to permit hands-free operation of the telephone. Such headsets incorporate an ear audio transducer which is held by the apparatus next to the user""s ear, and a microphone typically mounted to the end of a boom which extends forwardly from the ear piece to a position proximate to the user""s mouth. The ear audio transducer and the microphone are connected via wires to the telephone handset jack, either in substitution for or supplementary to the normal handset of the telephone.
A problem which is encountered whenever a new headset design is adapted for connection to a so-called xe2x80x9cnormalxe2x80x9d telephone handset, is that a large number and variety of commercially available handsets have been constructed with carbon microphones, condenser microphones, magnetic microphones, etc., all of which present different impedances to the telephone handset. Therefore, the handset transmission circuits have been designed in conjunction with the microphone type with which it is used, to provide the optimal impedance match for signal transmission. The impedance of different types of microphone varies considerably; for example, a carbon microphone has an output impedance of 150-400 ohms, a condenser microphone has an output impedance of about 1,500 ohms, and a magnetic microphone has an impedance somewhere in between that of a carbon and condenser microphone. Furthermore, a carbon microphone input circuit requires connection to the telephone handset voltage power supply and an electronic microphone is isolated from the telephone handset voltage supply. Therefore, if a headset adapter is to be designed for a particular type of handset, it becomes necessary to also select a microphone headset circuit to match the telephone handset input impedance, thereby requiring different headset designs to match different telephone handset circuits. It would be an improvement in the art if a single microphone circuit could be adapted to match a plurality of telephone handsets.
A further advantage which is sought in the design of interface circuits of the general type disclosed herein is to provide a low voltage, low current circuit capable of operation on the power provided by one or two xe2x80x9cAAxe2x80x9d size batteries. Such batteries provide 3.0 volts of power at full charge, and a slightly lower voltage level as the energy charge in the battery dissipates. It is advantageous to design an interface circuit capable of operating with signals within the 0-3 volt range, or slightly lower if battery discharge characteristics are taken into account. The prior art interface circuits usually operate from a voltage converter plugged into a wall outlet, which of course greatly increases the size and cost of such interface circuits.
It is a principal object of the present invention to provide a circuit which enables either of several microphone designs for a handset to efficiently couple to a telephone headset.
It is another object of the invention to provide a circuit capable of low power operation, and within voltage signal levels provided by batteries.
Other and further objects and advantages of the invention will become apparent from the following specification, and with reference to the drawings and the claims presented herein, which describe a preferred embodiment of the invention.
A circuit for matching the impedance of either of several types of telephone handset microphone input impedances to the impedance of a telephone headset. The circuit has a switchable input to connect either of two impedance matching circuits to a handset input. The first impedance match circuit includes an isolation transformer coupling, and the second impedance circuit includes a bridge circuit; the circuits operate at a very low voltage and current level, and are operable from low voltage batteries.